1. Field of the Invention
This invention relates to a drive circuit for a brushless motor including amplifiers energizing stator windings of the brushless motor through transistors of its output stage in accordance with a position detection signal generated by position detecting means detecting a rotational position of a rotor of the brushless motor, the amplifiers having an amplification degree controlled based on a motor speed control signal.
2. Description of the Prior Art
FIG. 4 shows an above-described type drive circuit for a three-phase brushless motor, for example. Position detecting elements 1a, 1b and 1c are provided for detecting a rotational position of a rotor 3 of the brushless motor. The position detecting elements 1a, 1b, 1c are connected to respective input terminals of a three-phase matrix circuit 2. Three amplifiers 4a, 4b and 4c are provided for supplying drive voltages to stator windings 3a, 3b and 3c of the brushless motor respectively.
Since the above-described amplifiers have the same electrical arrangement, only the amplifier 4a will now be described. The amplifier 4a is composed of a signal amplifying circuit 5a provided in an input stage, an NPN transistor 6a and a PNP transistor 7a both provided in an output stage. An input terminal of the signal amplifying circuit 5a is connected to an output terminal Pa of the three-phase matrix circuit 2. The signal amplifying circuit 5a has a control input terminal Ca to which a speed control signal S is delivered from a control circuit (not shown). The emitters of the transistors 6a, 7a are connected to each other such that the transistors 6a, 7a forms a series circuit. The collector of the transistor 6a is connected to a DC power supply terminal V.sub.CC and the collector of the transistor 7a is grounded. The bases of the transistors 6a, 7a are connected in common to an output terminal of the signal amplifying circuit 5a in common.
One terminal side of the stator winding 3a is connected to the emitters of the transistors 6a, 7a in common and the other terminal side of the stator winding 3a is connected to the other terminal sides of the other stator windings 3b, 3c in common.
In accordance with the above-described arrangement, the three-phase matrix circuit 2 delivers, to the amplifiers 4a-4c, voltage signals in accordance with the position detection signals supplied from the position detecting elements 1a-1c respectively. In this case, the delivered voltage signals each have a phase difference and a constant amplitude for the stator windings 3a-3c of each phase respectively. In each of the amplifiers 4a-4c, an amplification factor is varied in accordance with the speed control signal S so that a necessary electric power is supplied to the stator windings 3a-3c.
The transistors 6a-6c and 7a-7c in the output stages of the amplifiers 4a-4c deliver drive voltages V.sub.a, V.sub.b and V.sub.c in accordance with base signals supplied to them from the signal amplifying circuits 5a-5c respectively, whereby the brushless motor is driven at a set speed even when the torque applied to the motor varies.
Furthermore, in the above-described prior art arrangement, the drive voltages V.sub.a -V.sub.c are supplied to the stator windings 3a-3c by the linear operation of the amplifiers 4a-4c respectively such that rotation of the rotor is accelerated, and moreover, the electric power generated at the stator windings 3a-3c brings about a braking operation. In particular, the above-described arrangement presents fine characteristics when a load with large torque variations is driven and when a load is driven at a low speed with reduced torque ripples induced by the motor.
However, the conventional arrangement described above has the following disadvantage: the transistors 6a-6c and 7a-7c are linearly operated in the respective amplifiers 4a-4c. The transistors 6a-6c and 7a-7c share, as a voltage drop, the voltage obtained by subtracting the delivered drive voltages V.sub.a -V.sub.c from the power supply voltage V.sub.CC. In this case, an electrical loss W.sub.s depends upon the product of the above-mentioned voltage drop and a current I.sub.m flowing in the stator windings 3a-3c in the transistors 6a-6c and 7a-7c. Accordingly, the electrical loss W.sub.s is increased with a large rate of change toward a peak value with increase in the current I.sub.m, as shown in FIG. 5. Consequently, in the case where the brushless motor is driven under the condition that its rotational speed is controlled by the amplifiers 4a-4c, the transistors 6a-6c and 7a-7c of the respective amplifiers 4a-4c generate heat when the electrical loss W.sub.s is increased as described above, which prevents an efficient drive control.